High throughput printing systems, such as those used in high speed printers, color copiers, or large format devices, put heavy demand on an ink delivery system. The printhead must operate at a very high frequency. At the same time, print quality expectations keep rising. In order to maintain high print quality, the printhead must be able to rapidly eject ink without causing large fluctuations in the printhead pressure level.
One approach to this is to provide a pressure regulator integral to the printhead. The regulator receives ink at a first pressure and delivers ink to the printhead at a controlled second pressure. In order for this control to work, the first pressure must always be greater than the second pressure. Because of dynamic pressure drops, very high pixel rate printing requires that the first pressure be at a positive gauge pressure. Thus, the ink containers of such printers may be pressurized, such as with compressed gas, to forceably expel ink from the containers. An example of such a printing system is provided in U.S. Pat. No. 6,010,210, “Ink Container Having a Multiple Function Chassis.”
In printing systems where gas pressure is used to expel ink from a collapsible supply bag, a pressure drop is present across the bag which changes as the ink in the bag is depleted. As the bag is emptied, greater pressure is required to force the remaining ink out of the bag. The relationship between the pressure drop across the bag and the quantity of remaining ink is described in co-pending U.S. patent application Ser. No. 09/888,716, “Pressure Based Ink Level Detector and Method.”
When an ink bag in an ink supply has collapsed and is substantially empty and the gas pressure driving the ink through the ink lines is removed, a negative pressure may be created in the ink supply line from the ink container to the printhead. If this negative pressure exceeds the capillary pressure of the ink within the printhead, air may be drawn into the printhead through the printhead nozzles, causing printhead “deprime”. Subsequent use of the printhead can then result in degraded print quality or permanent damage to the printhead.
There are a number of prior art solutions to this problem. One such prior art solution includes stranding extra ink in the ink supply so that when the printer is turned off and the supplies are depressurized the negative pressure is not generated. This is a wasteful solution and is not operable for highly accurate ink level systems. Another prior art solution is to maintain the ink supplies close to the printhead in the Z direction, such that any negative pressure generated by the collapsing bag is countered by a pressure due to gravity. This solution works as long as there is sufficient room to keep the ink supplies at approximately the same elevation as the printhead. However, this solution typically requires a larger footprint for the printer.
A further prior art solution is to add a valve in the ink line and additional printer electronics and firmware to actuate a solenoid or motor to open and close the valve. When a motor is used, it is necessary to track position and, if the printer loses power, the valve may be left open. Using a solenoid still requires a separate actuation, so that it is possible for the valve to be left open or closed at the wrong time. If the valve is closed during printing due to a software or electrical defect, a set of very expensive printheads may be damaged or destroyed due to pen starvation, or “dry fire”. If the valve is left open when the printer is turned off, then printhead damage could occur due to the negative pressure generated by the ink supply.
An additional prior art solution is simply to use human actuation of ink valves. This solution would require the user to actuate the ink valves when removing the supplies. To prevent drawing air into the printhead, the method would require either the use of an idle pressurization of the ink supplies or keeping the supplies close in elevation to the printhead.
Thus, there is a need for apparatus and methods which reliably prevent negative pressure in a supply line from drawing ink out of a printer printhead when the printer is powered down.